Fibroblast Activation Protein (FAP) and Anti-FAP Antibodies
Human Fibroblast Activation Protein (FAP; GenBank Accession Number AAC51668), also known as Seprase, is a 170 kDa integral membrane serine peptidase (EC 3.4.21.B28). Together with dipeptidyl peptidase IV (also known as CD26; GenBank Accession Number P27487), a closely related cell-surface enzyme, and other peptidases, FAP belongs to the dipeptidyl peptidase IV family (Yu et al., FEBS J 277, 1126-1144 (2010)). It is a homodimer containing two N-glycosylated subunits with a large C-terminal extracellular domain, in which the enzyme's catalytic domain is located (Scanlan et al., Proc Natl Acad Sci USA 91, 5657-5661 (1994)). FAP, in its glycosylated form, has both post-prolyl dipeptidyl peptidase and gelatinase activities (Sun et al., Protein Expr Purif 24, 274-281 (2002)).
Human FAP was originally identified in cultured fibroblasts using the monoclonal antibody (mAb) F19 (described in WO 93/05804, ATCC Number HB 8269). Homologues of the protein were found in several species, including mice (Niedermeyer et al., Int J Cancer 71, 383-389 (1997), Niedermeyer et al., Eur J Biochem 254, 650-654 (1998); GenBank Accession Number AAH19190). FAP has a unique tissue distribution: its expression was found to be highly upregulated on reactive stromal fibroblasts of more than 90% of all primary and metastatic epithelial tumors, including lung, colorectal, bladder, ovarian and breast carcinomas, while it is generally absent from normal adult tissues (Rettig et al., Proc Natl Acad Sci USA 85, 3110-3114 (1988); Garin-Chesa et al., Proc Natl Acad Sci USA 87, 7235-7239 (1990)). Subsequent reports showed that FAP is not only expressed in stromal fibroblasts but also in some types of malignant cells of epithelial origin, and that FAP expression directly correlates with the malignant phenotype (Jin et al., Anticancer Res 23, 3195-3198 (2003)).
Due to its expression in many common cancers and its restricted expression in normal tissues, FAP has been considered a promising antigenic target for imaging, diagnosis and therapy of a variety of carcinomas. Thus, multiple monoclonal antibodies have been raised against FAP for research, diagnostic and therapeutic purposes.
Sibrotuzumab/BIBH1, a humanized version of the F19 antibody that specifically binds to human FAP (described in WO 99/57151), and further humanized or fully human antibodies against the FAP antigen with F19 epitope specificity (described in Mersmann et al., Int J Cancer 92, 240-248 (2001); Schmidt et al., Eur J Biochem 268, 1730-1738 (2001); WO 01/68708)) were developed. The OS4 antibody is another humanized (CDR-grafted) version of the F19 antibody (Wüest et al., J Biotech 92, 159-168 (2001), while scFv 33 and scFv 36 have a different binding specificity from F19 and are cross-reactive for the human and mouse FAP protein (Brocks et al., Mol Med 7, 461-469 (2001)). More recently, other murine anti-FAP antibodies, as well as chimeric and humanized versions thereof, were developed (WO 2007/077173, Ostermann et al., Clin Cancer Res 14, 4584-4592 (2008)).
Proteases in the tumor stroma, through proteolytic degradation of extracellular matrix (ECM) components, facilitate processes such as angiogenesis and/or tumor cell migration. Moreover, the tumor stroma plays an important role in nutrient and oxygen supply of tumors, as well as in tumor invasion and metastasis. These essential functions make it not only a diagnostic but also a potential therapeutic target.
Evidence for the feasibility of the concept of tumor stroma targeting in vivo using anti-FAP antibodies was obtained in a phase I clinical study with 131iodine-labeled F19 antibody, which demonstrated specific enrichment of the antibody in the tumors and detection of metastases (Welt et al., J Clin Oncol 12, 1193-1203 (1994). Similarly, a phase I study with sibrotuzumab demonstrated specific tumor accumulation of the 131I-labeled antibody (Scott et al., Clin Cancer Res 9, 1639-1647 (2003)). An early phase II trial of unconjugated sibrotuzumab in patients with metastatic colorectal cancer, however, was discontinued due to the lack of efficacy of the antibody in inhibiting tumor progression (Hofheinz et al., Onkologie 26, 44-48 (2003)). Also a more recently developed anti-FAP antibody failed to show anti-tumor effects in vivo in unconjugated form (WO 2007/077173).
Thus, there remains a need for enhanced therapeutic approaches, including antibodies with improved efficacy, targeting FAP for the treatment of cancers.
Antibody Glycosylation
The oligosaccharide component can significantly affect properties relevant to the efficacy of a therapeutic glycoprotein, including physical stability, resistance to protease attack, interactions with the immune system, pharmacokinetics, and specific biological activity. Such properties may depend not only on the presence or absence, but also on the specific structures, of oligosaccharides. Some generalizations between oligosaccharide structure and glycoprotein function can be made. For example, certain oligosaccharide structures mediate rapid clearance of the glycoprotein from the bloodstream through interactions with specific carbohydrate binding proteins, while others can be bound by antibodies and trigger undesired immune reactions (Jenkins et al., Nature Biotechnol 14, 975-81 (1996)).
IgG1 type antibodies, the most commonly used antibodies in cancer immunotherapy, are glycoproteins that have a conserved N-linked glycosylation site at Asn 297 in each CH2 domain. The two complex biantennary oligosaccharides attached to Asn 297 are buried between the CH2 domains, forming extensive contacts with the polypeptide backbone, and their presence is essential for the antibody to mediate effector functions such as antibody dependent cell-mediated cytotoxicity (ADCC) (Lifely et al., Glycobiology 5, 813-822 (1995); Jefferis et al., Immunol Rev 163, 59-76 (1998); Wright and Morrison, Trends Biotechnol 15, 26-32 (1997)). Protein engineering studies have shown that FcγRs interact with the lower hinge region of the IgG CH2 domain. Lund et al., J. Immunol. 157:4963-69 (1996). However, FcγR binding also requires the presence of the oligosaccharides in the CH2 region. Lund et al., J. Immunol. 157:4963-69 (1996); Wright and Morrison, Trends Biotech. 15:26-31 (1997), suggesting that either oligosaccharide and polypeptide both directly contribute to the interaction site or that the oligosaccharide is required to maintain an active CH2 polypeptide conformation. Modification of the oligosaccharide structure can therefore be explored as a means to increase the affinity of the interaction between IgG1 and FcγR, and to increase ADCC activity of IgG1s.
A way to obtain large increases in the potency of monoclonal antibodies, is to enhance their natural, cell-mediated effector functions by engineering their oligosaccharide component as described in Umaña et al., Nat Biotechnol 17, 176-180 (1999) and U.S. Pat. No. 6,602,684 (WO 99/54342), the contents of which are hereby incorporated by reference in their entirety. Umaña et al. showed that overexpression of β(1,4)-N-acetylglucosaminyltransferase III (GnTIII), a glycosyltransferase catalyzing the formation of bisected oligosaccharides, in Chinese hamster ovary (CHO) cells significantly increases the in vitro ADCC activity of antibodies produced in those cells. Overexpression of GnTIII in production cell lines leads to antibodies enriched in bisected oligosaccharides, which are generally also non-fucosylated and of the hybrid type. If in addition to GnTIII, mannosidase II (ManII) is overexpressed in production cell lines, antibodies enriched in bisected, non-fucosylated oligosaccharides of the complex type are obtained (Ferrara et al., Biotechn Bioeng 93, 851-861 (2006)). Both types of antibodies show strongly enhanced ADCC, as compared to antibodies with unmodified glycans, but only antibodies in which the majority of the N-glycans are of the complex type are able to induce significant complement-dependent cytotoxicity (Ferrara et al., Biotechn Bioeng 93, 851-861 (2006)). Alterations in the composition of the Asn 297 carbohydrate or its elimination also affect binding of the antibody Fc-domain to Fcγ-receptor (FcγR) and complement C1q protein, which is important for ADCC and CDC, respectively (Umaña et al., Nat Biotechnol 17, 176-180 (1999); Davies et al., Biotechnol Bioeng 74, 288-294 (2001); Mimura et al., J Biol Chem 276, 45539-45547 (2001); Radaev et al., J Biol Chem 276, 16478-16483 (2001); Shields et al., J Biol Chem 276, 6591-6604 (2001); Shields et al., J Biol Chem 277, 26733-26740 (2002); Simmons et al., J Immunol Methods 263, 133-147 (2002)).